1. Field of the Invention
The present invention relates to processes for producing deep-drawing, non-ageing cold rolled steel sheets having excellent press formability and paint bake-hardenability.
2. Description of the Prior Art
Press-forming cold rolled steel sheets and strips (hereinafter called "strips") used in automobile cars are required to have excellent deep-drawability, stretchability, shape quality and non-ageing property, and these requirements are particularly important for use in the outer skin applications, such as doors, roofs and quater pannels.
Moreover in recent years, for the purpose of obtaining a high dent-resistance of pannels due to the car vibration, increasing demands have been made on the strips for an additional property, called "paint bake-hardenability" that the yield point of the steel strips can rise remarkably during the heat treatment for paint baking on the steel strips in the automobile car production.
Cold rolled steel strips having such paint bake-hardenability are known, as disclosed in Japanese Patent Application Laid-Open No. Sho 54-107419, according to which Al-killed steels are subjected to hot and cold rollings, then subjected to an open coil annealing wherein the strips are soaked at a temperature ranging from A.sub.1 point to A.sub.3 point, and cooled at a cooling rate of 30.degree. to 200.degree. C./hour, or Al-killed steels having a lowered carbon content of about 0.01% are subjected to a tight-coil box-type annealing so as to increase the solid solution carbon. However, the paint bake-hardening degree obtained by this prior art is still far below 5 kg/mm.sup.2 which is an ordinary standard for the purpose. Moreover, the annealing in the prior art is done by the box-type annealing process which comprises slow cooling, long-time soaking, and slow cooling, so that a considerably long time is required, thus causing problems with respect to the productivity.
Meanwhile, several proposals have been made as disclosed in Japanese Patent Publications No. Sho 47-33409 and No. Sho 49-1969 for production of cold rolled steel strips having an excellent press formability, such as deep-drawing and stretchability, and their production has limitedly been made on a commercial scale.
However, according to these prior arts, it is essential to heat and soak the steel in a continuous annealing furnace, then rapidly cool the steel to about 400.degree. C., for example, and overage the steel near this temperature, or to cool the steel to the room temperature, then reheat the steel to about 400.degree. C. and overage the steel near this temperature.
These conventional cold rolled steel strips obtained by continuous annealing have a problem that the yield point elongation appears so far as they are in "as non-skinpassed state", namely they are ageing, even if they have been overaged, or even if they have lowered C and N contents and contain additional elements, such as Al and B.
Therefore, these prior arts cannot satisfactorily produce a deep-drawing, non-ageing, cold rolled steel strip having an excellent paint bake-hardenability as desired by the present invention.
Also according to the conventional continuous annealing process, it is essential to perform the overageing treatment as mentioned before in order to reduce the solute C and N, so that the production cycle can be shortened only limitedly and the continuous annealing line must be considerably long.